Modern integrated circuits include several levels of metallization formed over the active semiconductor circuitry formed in the substrate, most commonly a silicon wafer. The multiple metallization layers are required to interconnect the millions to tens of millions of discrete semiconductor devices formed in a single integrated circuit. For advanced microprocessors, there may be five or more levels of metallization.
Each metallization layer includes a dielectric layer, for example, of silicon dioxide or possibly a low-k dielectric material deposited over the previous layer, a via (or contact) hole etched through the dielectric layer, and a metallization including a vertical metallic plug filled into the hole and a horizontal interconnect formed on top of the dielectric layer.
The formation of vias has become increasingly crucial with the continuing shrinkage of critical dimensions and the transition to copper rather than aluminum interconnects. Contacts to underlying silicon will for the most part not be explicitly described hereafter but can equally benefit from the various aspects of the invention.
A generic via is illustrated in the cross-sectional view of FIG. 1. It includes a lower dielectric layer 12 (which in the case of a contact is a silicon-containing layer) having formed therein a conductive feature 14, such as another metallization or, in the case of a contact, a contact region of a transistor. An upper level dielectric layer 16 overlies the lower dielectric layer 12 and the conductive feature 14. Advanced plasma etching processes etch a via hole 18 through the upper dielectric layer 16 to the vicinity of the conductive feature 14. This explanation avoids many details about etch stop layers, horizontal interconnects, and advanced structures, particularly including dual damascene, but the fundamental concepts for the invention remain the same.
Prior to filling metallization into the via hole 18, the hole is lined with a barrier layer 20 to prevent the diffusion of aluminum or copper into the dielectric 16, which would short out the dielectric between neighboring via, or the diffusion of oxygen from the dielectric 16 into the metallization, which reduces the conductivity of the plug. Only thereafter is a metallization plug 22 filled into the via hole 18. In the case of copper metallization, the filling process typically includes a physical vapor deposition or sputtering (PVD) deposition of a copper seed layer followed by an electrochemical plating (ECP) of copper into the hole.
The barrier structure has become increasingly important as the lateral critical dimension has shrunk to 0.25 μm and below while the vertical dimension has remained virtually static at between 0.7 and 1.51 μm. As a result, the aspect ratio of the via hole 18 has increased, and the electrical resistance of the vertical electrical metallization has become increasingly important. This emphasis on reduced vertical impedance has been further heightened by the substitution of copper for aluminum as the most desired metallization since copper has a lower bulk resistivity. However, the controlling parameter is the total resistance along a path, including the resistance through the bottom barrier layer portion 24 at the bottom of the via 18. Particularly in the case of an inter-metal dielectric between two metallizations of the same metal, there is no need to include the bottom barrier layer portion 24 since the copper or other metal in the via 22 will be contacting a same metal in the conductive feature 14.
In U.S. Pat. No. 5,985,762, Geffken et al. have disclosed directionally etching away the barrier layer exposed on horizontal surfaces of a copper dual-damascene structure but leaving the barrier layer on the sidewalls to protect the dielectric sidewalls from copper sputtered from the underlying copper feature. This process requires presumably a separate etching chamber. Furthermore, the process deleteriously also removes the barrier at the bottom of the trench in a dual-damascene structure. They accordingly deposit another conformal barrier layer, which remains under the metallized via.
In commonly assigned U.S. patent application Ser. No. 09/518,180 filed Mar. 2, 2000, now issued as U.S. Pat. No. 6,277,249, Gopalraja et al. have suggested that a self-ionized plasma sputter deposition of a copper seed layer for a copper metallization 22 can be used to remove the bottom barrier layer portion 24.
Accordingly, it is desired to provide a structure and method of making it which reduces the contact resistance at the bottom of a via.
It is further desired to provide such structure and method without unduly complicating the integrated circuit fabrication process.